1. Field of the Invention
The present invention relates to a flow rate control system to be adopted to the apparatus which supplies or drains, for example, gas and liquid such as air and water as the fluid and more specifically to a system which controls a flow rate of fluid to be supplied or drained by a fluid flowing means through the control of a number of revolutions of a motor which drives the fluid flowing means such as a fan which defines the fluid as a load and causes this fluid load to flow.
2. Description of the Prior Art
In an ordinary fluid supply and drain apparatus providing a blower or a pump, etc., a flow rate of fluid to be supplied or drained can be controlled by driving a load of fan or pump with a driving means such as a motor, controlling this driving means and adjusting a flow rate control means such as a damper, valve and static wing, etc.
As an apparatus and a method for controlling a load and a driving means for the load, there is known prior art. One was filed as an application on Feb. 2, 1983 and is described in the Published Japanese Patent No. 58-6077 and the other is described in the U.S. Pat. No. 4,181,099 (Jan. 1, 1980).
The patent entitled, "Method for Operating a Load such as Fan to be Driven by a Motor" described in the Published Japanese Patent No. 58-6077 utilizes a flow rate control system shown in the block diagram of FIG. 1 and explained below. In FIG. 1, 1 is a commercial power supply. 2 is a variable frequency power supply (VVVF). 3, 4 are switches. 5 is a motor. 6 is a fan as a means which supplies a fluid such as air. 7 is a rotation detector which detects a number of rotations of a motor such as a specified speed generator.
Operations are then explained. A synchronous motor is used as a motor 5 and it is supposed that VVVF 2 copes with a load up to the amount of wind at a speed of 80%. In this case, the switch 3 is opened and the switch 4 is turned ON for the flow rate up to 80% speed and moreover the motor 5 is driven with VVVF 2. When the flow rate of 80% speed or more is required, a damper as a fluid control means is closed in order to keep light the load of motor 5 and the frequency of VVVF 2 is raised up to a value a little higher than that of the commercial power supply. In this case, an armature voltage is kept constant by field control. When the frequency of motor 5 reaches a point a little higher than the commercial frequency, the switch 4 is opened, the synchronous field is enhanced and set to the commercial power supply voltage and the switch 3 is turned ON when the outputs of commercial power supply 1 and motor 5 are matched in the phase within the allowable error. Thereby, the input of motor 5 can be switched to the commercial power supply 1 from VVVF 2 without an excessive transient phenomenon.
Meanwhile, in case the flow rate of 80% speed or less is required again, the switch 3 is opened in order to reduce a number of rotations of motor 5. The frequency of VVVF 2 is changed synchronously with a signal of the detector 7 which detects a number of rotations of motor 5 and it is operated synchronously with the output frequency of motor 5. When the number of rotations becomes lower than 80% speed the switch 4 is turned ON. Thereby, the motor 5 can be connected with VVVF 2. This operation can be employed even when an induction motor is used as the motor 5.
In case the motor is controlled as explained above, the relation between fan input and the number of rotations of fan is indicated by the characteristic of FIG. 2. Effective energy saving can be realized by saving the fan input indicated as the hatched region in FIG. 2.
The flow rate control system shown in FIG. 3 is proposed to more practically realize the method for operating a fan, etc. described above.
In the block diagram of FIG. 3, 11 is a commercial power supply. 12 is a switch. 13 is a variable ferquency power source (VVVF). 14 is a fan motor. 15 is a fan (including a vane). 16 is a coupling axis. 17 is an air duct for air moved by the fan 15. 17a, 17b are an inlet and an outlet respectively, for air flowing through the air duct 17. 18 is a damper as a flow rate control means housed in the air duct 17. 19 is a driving rod for damper 18. 20 is a damper driving apparatus. 21 is a control apparatus which generates a control input to VVVF 13 and damper driving apparatus. 22 is a transformer. 23 is a detector for number of rotations. 24 is a load detector. 23a, 23b are contacts which are actuated OFF and ON when the detector 23 operates. 24a, 24b are contacts which are actuated OFF and ON when the load detector 24 operates. 25 is a fixed control input giving a signal which makes constant the output of VVVF 13. 28 is a control input terminal to the driving apparatus 20.
Operations are explained. The fluid flow control apparatus keeps a rotating speed of fan motor 14 for the present flow rate Q.sub.L so that the speed does not go below a certain number of rotations N.sub.L and the opening angle of damper 18 is kept small as shown in FIG. 4 for a flow rate lower than Q.sub.L. The practical control is carried out in the following way. Namely, an output frequency f and an output voltage V of the variable frequency power supply 13 are in a functional relation. Therefore, when a frequency f is determined, an output voltage V is also determined uniquely. In other words, the equation (1) can be obtained. EQU V=g(f) . . . (1)
Where, g: function.
Usually, the equation (2) is used in order to avoid saturation of the winding of the motor. EQU g(f)=K f . . . (2)
Where, K is constant.
From above explanation, it is apparent that an output frequency f can be detected from an output voltage V of variable frequency power supply 13. Therefore, a detector for a number of rotations 23, which detects a frequency, can be employed and therefore a number of rotation N below the constant value N.sub.L can be obtained from the equation (1) by changing an output voltage V of the variable frequency power supply 13 to a value suitable for measurement through transformer 22 and detecting a value lower than the constant value.
When the number of rotations N is higher than a constant value N.sub.L, the number of rotation detector 23 does not operate and therefore the load detector 24 also does not operate and thereby the contacts 24a, 23b are OFF, while the contacts 23b, 24b are ON. Therefore, in this case, a control signal of the control apparatus is applied to the variable frequency power supply 13, while a control input is applied to the damper driving apparatus 20. A damper opening angle .theta..sub.c in the vicinity of full opening can be obtained for the range of flow rate of Q.sub.L .about.100% in FIG. 4.
When the number of rotations N attempts to go below a constant value N.sub.L, the detector 23 operates, causing the contact 23a to become ON and the contact 23b OFF. When the contact 23a becomes ON, and load detector 24 becomes operative, but since setting is made so that it becomes inoperative when flow rate Q is smaller than the flow rate Q.sub.L corresponding to a number of rotations N.sub.L, the contacts 24a, 24b are respectively OFF, ON. Accordingly, a fixed control input is applied to the variable frequency power supply 13 and the number of rotations is fixed at N.sub.L and a control input of the control apparatus 21 is given to the damper driving apparatus 20. In the range lower than the flow rate Q.sub.L, an output frequency f.sub.L of the variable frequency power supply 13 is fixed to a value indicated by the equation (3), and it is shown in FIG. 4(a). EQU f.sub.L =N.sub.L .times.(P/120) . . . (3)
When a control signal of control apparatus 21 is given to the damper drive apparatus, the damper 18 operates providing the requested flow rate and the damper 18 is squeezed in a range lower than the flow rate Q.sub.L as shown in FIG. 4(b). Next, the method for returning the opening angle of damper to the original opening .theta..sub.C when the flow rate Q exceeds Q.sub.L from the condition described above is explained hereunder.
When the rotation detector 23 is operating, and the contact 23a is ON the the load detector 24 is operable. When the requested flow rate increases and the control signal of control apparatus 21 increases while an opening angle of damper reaches .theta..sub.C, the load detector 24 operates, making the contacts 24a, 24b ON and OFF. Therefore, a control input is applied to the damper driving apparatus 20 so that it is fixed at the damper opening angle .theta..sub.C and the signal of control apparatus is applied to the variable frequency power supply 13. When the frequency f further increases, the rotation detector 23 restores to the initial condition of flow rate Q.sub.L .about.100%.
However, the existing flow rate control systems described above have the problems to be explained below.
First, safe operation is assured for entire range of load but investigations for variations of comparatively small load are not yet carried out. For example, a large supply current from VVVF 2 is necessary to increase the velocity for increase in load while the VVVF 2 is operating under the 80% load in FIG. 2 and the capacitance of VVVF 2 must be increased as much as such amount. Meanwhile, for reduction in load, rotating speed of the motor 5 operated by VVVF 2 is naturally reduced and a response rate is low for the load having large value of GD.sup.2 such as a fan 6. When a recovery braking function is added to improve such response rate, VVVF 2 becomes large in size and also becomes expensive.
Even in case the flow rate control system explained with reference to FIG. 3, FIGS. 4(a)(b), the fan motor 24 is controlled in such a manner that it continuously changes from N.sub.L to 100% rotation region. Therefore, if a resonant point of mechanical oscillation frequency of the fan 25 lies in a certain region of the continuously variable frequency, mechanical oscillation is amplified, resulting in a problem such as failure of fan 25.
Moreover, the existing flow rate control system contains a problem that the energy saving effect is lowered in comparison with that where the damper is used under the full opening condition because the damper is operated in the direction of opening for increase of load as the assistance for quick control response since the control of opening of the damper 18 is also employed in parallel and therefore opening of damper must be set to about 50.degree. in order to improve controllability.